44 SECRETS OF ANIMAL LIFE 
They are adapted to their curious history in being 
relatively small (though with considerable variabil- 
ity in size) and in having thick resistant shells, but 
still more strikingly in being, in many cases, like 
facsimiles of the eggs of the selected foster-parent. 
The fine “Fenton Collection” of birds’ eggs in the 
University of Aberdeen has over four hundred 
cuckoo’s eggs in the clutches of over fifty different 
kinds of foster-parents, and the two immediate 
impressions that one gets are, first, that the cuckoo’s 
egg is often a perfect copy of those of the foster- 
parents; and, second, that it is often obtrusively 
conspicuous. Now it seems to be practically certain 
that the same cuckoo lays the same type of egg con- 
sistently, and it is probable that Professor Newton’s 
theory is right, that having a blue egg, for instance, 
may be hereditary in a given lineage, and that there 
may also be in the same lineage a hereditary pre- 
disposition to put the egg in a Redstart’s blue 
clutch. If the cuckoo is hurried or flurried, it may 
put the egg in a clutch with which it is inharmoni- 
ous, and as this often succeeds perfectly well, it 
seems likely that some kinds of birds are much less 
sensitive than others to the presence of an intruded 
egg. Thus it is very rarely that a blue Cuckoo's 
egg is found in the blue clutch of the Hedge- 
Sparrow, and there is no “mimicry” between a 
Cuckoo’s egg and a Wren’s. 
Another of the major cuckoo-puzzles has to do 
with the behavior of the young bird in the nest. 
What Jenner observed so carefully in 1788, several 
